The honest answer is never. You can put things on various schedules to strengthen behavior without having to reward every single time. However, I don't know a lot of folks doing this that aren't already dog trainers.
The truth is, behavior is a product of consequences. If we want certain behaviors, we have to provide incentive.
A great example of the power of rewarding each time is recall. I advise my clients to reward their dogs for coming when called every single time. In the dog park, in the yard, even inside the house. Don't just reward every time, reward liberally. Reward impressively. Use cheese and salami and tripe - and lots of it! Why? Because one day that recall could save your dog's life.
One day someone might leave the front door wide open and all you see is your dog running toward the busy intersection. You've got a much better chance of getting your dog back if you've always, always, always rewarded that recall. That's the day you'll be glad you weren't skimpy with the treats.
Reward every time in preparation for the times that you can't. For example, my dog recently had a dental cleaning for which she had to be put under general anesthesia. The morning of her dental work, despite being unable to feed her, I was able to perform our usual routine of cleaning the sensitive skin on her stomach and giving her her daily medications. I was able to do it because we had had plenty of practice.
When it comes to building a reliable behavior, there are of course other variables to consider. The following will aid you in strengthening your dog's learned cues:
1) Help your dog be less literal.
Reward when she isn't expecting it. Keep treats stashed around your home so you can use them at a moment's notice. Use life rewards like opening the car door, going for a walk or allowing your dog to chase that squirrel.
One effective way to help your dog be less literal is the "bridge and fridge" technique. This technique allows you to not always have treats at the ready in a pouch or pocket. It's very simple. Once your dog performs the desired behavior, you begin enthusiastically praising her while running to the fridge for her favorite goodies. By doing this, you're teaching your dog that just because you don't have treats on your person does not mean that treats won't happen.
2) Be less predictable.
Mix it up. Do one practice trial a day. Do it at random times when your dog isn't expecting it. The less it looks like a training set up, the better.
It's important to remember that providing desirable consequences for behavior is all about increasing the probability that it will happen in the future. We are putting money in the bank and tipping the scales in our favor. However, we should always keep in mind that there are no guarantees in behavior.
The day that your dog runs out the front door, there might be an entire family of squirrels down the street. There is no guarantee that your accrued interest will outweigh the fun provided by the squirrels. But, if you can spend time making it more likely, why wouldn't you?
The situations like these that are beyond our control are scary. Please always play it safe and manage these situations to the best of your ability. Wether this means closing the door, leashing your dog or using a baby gate for your door dasher, safety is paramount! In the meantime, work on making yourself a source of all good things to your dog, you'll be glad you did.